The incidence and complexity of electronic circuitry continues to increase, requiring improvements in the efficiency and functionality of the test equipment necessary to diagnose and repair such circuitry. Test equipment such as multi-meters and logic analyzers may generally communicate with the circuitry being tested via one or more conductive test probes. Test probes may be configured to pierce an insulated wire or contact an exposed electrical terminal. Measured parameters may include resistance, voltage, current, impedance, frequency, and digital waveforms. In addition, some multi-meters, such as the multi-meter disclosed in U.S. Pat. No. 7,184,899, have a “power and measure” feature, allowing a technician to probe a circuit, initiate current sourcing, and simultaneously measure electrical parameters, thus enhancing test efficiency. Some important quality goals for test equipment may include collecting accurate measurements at the required resolution without interfering with the operating point of the circuit under test. Unfortunately, certain design aspects of conventional test equipment may result in poor measurement quality when diagnosing a circuit.
For example, one cause of poor measurement quality may be a test probe resistance that is higher than the required measurement resolution. For example, a test probe resistance of 100 milliohms (mΩ) would interfere with the accurate measurement of a circuit resistance requiring a resolution of 10 mΩ by adding 100 mΩ to the measurement result. Test probe resistance is measured across the conducting length of the test probe, and does not include the contact resistance occurring between the test probe and the circuit node. Test probe resistance may depend on the resistivity of the conductor used, on corrosion, and on any damage that occurs due to misuse or high current flow. Often, the test probe may be fabricated from a metal material having a relatively low resistivity, which may be adequate for conducting moderate current flows of less than approximately 1 to 10 amps. For example, brass, an alloy of copper and zinc, has a resistivity of approximately 6-7 μΩ-cm, where metals in general may range from 1-300 μΩ-cm in resistivity. However, many automotive circuits require testing with high current flows of from approximately 10 to 100 amps or more, which may require the alteration of the geometry or diameter of a brass or steel test probe to insure that test probe resistance remains small.
Another cause of poor measurement quality may be electrical arcing that may occur between the test probe and the circuit node being measured. Arcing may generally begin during the making or breaking of contact involving a live circuit in close proximity. A live circuit may occur by either placing a test probe onto an energized circuit node having a voltage, or by placing an energized test probe onto a non-energized or passive circuit node, or both. A sustained arc having sufficient heat energy, i.e., sufficient current, may melt steel. The effect of even momentary arcing may be a break down or damage in the conductor material such as brass or steel, causing heating, burning, melting, material migration, molecular voids, and a permanent rise in test probe resistance that begins to interfere with the quality of the measurement. Arcing may self-extinguish once the distance between electrical nodes increases. However, the availability of larger current flows may increase the heat energy within an arc, increasing the possibility of material damage. A test probe susceptible to arc damage may degrade progressively over time, attracting larger and longer arcs, and increasing test probe resistance further. Conventional metallic test probes may therefore require frequent replacement.
Another cause of excessive test probe resistance may be a temporary heating of the sharpened tip through either arc heating or resistive heating. During arc heating, initiating an electrical contact may give rise to a momentary arc over the sharpened tip of the test probe, heating the surface of the probe tip. Since many metals, such as brass and steel, possess positive resistive temperature coefficients, their resistivity may increase upon the occurrence of an arc. A multi-meter measurement immediately following such an arc may experience increased test probe resistance that gives rise to measurement error. In addition to arc heating, a substantial current flow through the sharpened tip during a measurement may give rise to resistive heating that also creates increased resistivity, building on any preceding arc heating and causing more measurement error. For example, a sharpened tip comprised of a metal having a positive temperature coefficient may be required to conduct the flow of 100 amps through a very small volume of metal, generating heat and increased resistance, and subsequently measurement error. For example, a 100° C. increase in the temperature of the tip of a copper test probe having a temperature coefficient of 0.0043/° C. causes tip resistance to increase by 43% (i.e., 100×0.0043), and causing an increase in overall test probe resistance.
Another problem that may arise during measurements involving a conductive test probe relates to the arc detection circuitry disclosed in the above-mentioned application Ser. No. 13/404,644. During arc detection, high frequency spectra may be analyzed for evidence of arcing internal to the circuitry being powered and measured by the test equipment. The test may be terminated by a circuit breaker upon detection of an internal arc. The accuracy of the measurement depends on a fast rise time in the voltage or current, which requires a small test probe resistance. However, the occurrence of test probe arcing may interfere with the internal detection of arcing by interfering with the sense threshold causing arcing to increase before the sense threshold is achieved. Additionally, as the test probe resistance increases further and the rise time gets progressive slower, the response may eventually become so delayed that the circuit breaker does not shut down at all, posing a fire hazard to a circuit that is external arcing.
A further cause of excessive test probe resistance may be the use of test probe geometries that generate high resistive losses. For example, the sharpened tip of the test probe may be the location of highest current density and may thereby contribute excessively to the overall test probe resistance if there is a long path from the probe tip to a conductive portion of the test probe having relatively low current density. In addition, local heating may occur near the tip due to current flow, further increasing path resistance.
As can be seen, there exists a need in the art for a durable conductive test probe that can withstand arcing over many cycles under high current conditions. Additionally, there exists a need in the art for a test probe that maintains a low resistance at high currents during arcing so that measurement resolution and accuracy are maintained. Furthermore, there exists a need in the art for a test probe that preserves the rise time of arc detection circuitry by preventing an increase in test probe resistance caused by arcing. There also exists a need in the art for a test probe that can withstand many contact cycles under live circuit conditions. In addition, there exists a need in the art for a conductive test probe that has a compact (i.e., small) geometry that minimizes test probe resistance.